Interpretation of seismic records



Jan. 16, 1962 R. M. ALLYN ETAL INTERPRETATION oF sETsMTc RECORDS 6 Sheets-Sheet 1 Filed NOV. 17, 1958 RECORDER Il Isch 13ml lscw i3d, I3@N |35 LLJ INVENT ORS s. D. ELL|OTT R. M. ALLYN Hum @WM5 AT TOR/VE' YS 6 Sheets-Sheet 2 Filed Nov. 17, 1958 All AwOZOUmwV MEE.. ON m. m;

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INVENTORS S.D. ELLIOTT R. M. ALLYN ATTORNEYS R 6 Sheets-Sheet 3 Filed Nov. 17, 1958 WMMpVMmW/v MMJW-/WWWJ TIME (S ECONDS) INVENTORS S. D. ELLIOTT R. M. ALLYN BY H im T (3M A TTORNEVS Jan. 16, 1962 R. M. ALLYN ETAL 3,015,970

INTERPRETATION OF SEISMIC RECORDS Filed Nov. 17, 1958 6 Sheets-Sheet 4 INVENTORS S. D. ELLIOTT R. M. ALLYN BYHJAMAXM A TTORNEYS Jan. 16, 1962 R. M. ALLYN ETAL INTERPRETATION OF SEISMIC RECORDS Filed NOV. .'17, 1958 6 Sheets-Sheet 5 A T 7'ORNE V5 Jan. 16, 1962 R. M. ALLYN ETAL 3,016,970

INTERPRETATION OF SESMIC RECORDS Nmvvwww L6 l.7 L8 L9 2.0 2.l 2.2 2.3 2.4

A TTORNEVS Uted States Patent 3,016,970, INTERPRETATION F SEISMIC RECORDS Robert;M. Allyn.l and Sami); Elliott, Bartlesville, Okla., assignorsto Phillipsletroleum Company, a corporation of. Delaware Filed Nov. 17, 1958, Ser. No. 774,334 3 Claims. (Cl. 181-5) rI `his invention relates to a method of seismic exploration wherein a plurality of separate vibration records obtained from va common subterranean reflection pointV or l areaV are added in a manner so as to amplify the desired reflections and minimize extraneous noise vibrations Seismic exploration relates to a rmethod of obtaining information regarding subterranean earth formations by transmitting vibrations from a first point at or near the 3,016,970 Patented Jan. 16, 1962 2 1 inv particular, there is shown a seriesI of spaced shot holes a, 10b, 10c, 10d, 10e and 10]c which are positioned along a common line.. A detonatorl 11 is connected to an explosive charge 12 which is positioned withsurface of the earth downwardly intothe formations and measuring the reflected or refracted vibrations at one or more second points spaced from the first point. It is com-` mon practice to detonate an explosive charge to produce the vibrations. A plurality of seismometers are disposed in a predetermined geometric array in spaced relationshipy from the shot hole. The vibrations incident upon the seismometers are converted into counterpart electrical regions. A- portion of theenergy transmitted downwardly is reflected'back to the surface from the lower boundaryI of' this weathered layer; rIhis reflected energy is partially transmitted downwardly againrat the surface, and these reflections continue until al portion of the energy reachesl the seismometers. In .some areas, these multiple reflections often arrive'at the seismometers withgreater amplitudes than do reflections from lower boundaries which are to he mapped. It thusbecornes important to eliminate these multiple reflections from the seismic records.

It has recently been proposed to combine `a plurality Of-seismic records in such a manner that reflections from common subterranean formations are` superimposed in thecompositeV record. The present invention provides an improved procedure' of this type wherein the individual records are displaced from one another andsubsequently combinedfso that the mutliple reflections are cancelled andthe desired reflections `are amplified.

Accordingly, it is i an object of this invention to `provide a procedure for recording and reproducing seismic signals in such a manner as to amplify the desired reflections and minimiz'elrandom vibrations. Y'

Another object i`s to provide a procedure for transforming` seismic records so as to eliminate ymultiple reflections. l

Other objects, advantages and features'of the invention should become apparent from the following detailed description which is taken in conjunction with the accompanying drawing in which:

FIGURE 1l is a schematic representation of the locations of shot points andtseismometers which can be em-v in shot hole 10a. A series of seismometers 13a, 13b, 13e, 13d, 13e and 13f are spaced from one another and* from the shot holes along the same line. These seismometers can represent single seismometers, or groups of seismometers, as is well known in the art. The seismometers are connected to a recorder 14 which preferably comprises a multi-channel magnetic drum type recorder. This type of recorder is preferredbecause the signals can readily be reproducedfor transformation in accordanceV with this invention. f

An unconsolidated or weathered layer 15 is shown immediately beneath the surface of the earth. A rock bed 16 is shown a substantial distance beneath this weathered layer. When explosive charge 12 is detonated,v

vibrations are transmitted downwardly and are reflected from bed 16 back to seismometer 13a. Additional vibra tions are reflected a number of times through the weathered layer, as illustrated, tostrike seismometer 13a.. It

should thus be evident that the signal recorded from this` seismometer contains both the desired reflections from bed 16 and multiple reflections through the weathered layer.

Additional explosive charges are detonated in sequence in the remainder of the shot' holes and vibrations are received by the corresponding seismometers. For. example, vibrations ernitted'from shot hole lllf are received by seismometer 13f. In actual practice, vibrations from each shot hole arereceived by a plurality of seismometers.

However, in order to simplify the explanation of this iny vention, it will be assumed that seismometers 13a to 13f receive onlyjvibrations from respective shot holes 10a toV 10j. It should be noted that these vibrations are rellected froma common region on bed 16;

FIGURE 2V is a graphical representation of typical sig- I nals which are recorded from vibrations received by the individualseismometers. Curves 20a to 20f represent vibrations received by-'respectlve seismometers 13a to 13f. In these curves, it is assumed that reflections from bed'16 occur at times between 1.7 and 1.9 seconds after the respective explosive charges are detonated. These reflections aredesignated by the letter R on each of the curves. It is also assumed thatr the multiple reflections arrive atV times between 2.2 and 2.4 seconds after the respective explosive charges are detonated. These multiple reflections are designated by the letter M. lt is furtherassumed that the weatheredV layer transmits vibrations at` an average velocity of approximately 6500 feet per second, whereas the average, velocity of transmission through the lower regions is approximately 9,000 feet per second. The horizontal distances from respective shot holes 10a to 101 tol the corresponding seismometers o and -900'feet,r1800 feet, 2700 feet, 3600 feet, 4500 feet and 5400-feet, respectively.

From an inspection of FIGURE 1, it is evident that the vibrations which travel from shot hole 10j to seismometer 131 travel a longer path thando the vibrations from shot hole ,10a which are receivedlby sesmometer 13a; The llrst' step in the process of this invention involves combining the several records so that all the re tlections'fromr bed 416 are superimposed. This requires that the individual records be displaced by varyingl `amounts to correct for the different travel paths. The

3 ly are recorded on a magnetic drum. For any given assumption of the depth of the reflecting bed 16, and thus the travel time for a primary reflection, the corresponding displacements for the individual seismometer signals can readily be computed from the horizontal spacings 4. previously applied. The resulting curves 25a to 25), together with the displacement times, are illustrated in FIG- URE 5. Curves 25a to 25f are then summed and reduced in amplitude by a factor of one-sixth to provide curve 26. The final step of the process of this invention between the seismometers and the shot pomts and the involves subtracting curve 26 from curve 22 of FIGURE' average velocity of transmission through the formations. 3. This is readily accomplished by inverting curve 26 These transmission velocities generally can be obtained to provide curveZo' which is then summed with curve 22 by procedures well known in the art, such as by drilling to provide curve 27. From an inspection of curve 27 an exploratory well and detonating a series of charges in FIGURE 5, it can readily be seen that the primary at various depths. The table which follows shows the reflections appear in the record, whereas the multiple angularity of path correction times to be applied for varirellections are substantially eliminated. ous assumed depths of the reilecting bed 16. A second embodiment of the process of this invention Horizontal Distance-Shot Point to Geophone (Feet) Angularity of Path Correction (Seconds) Each horizontal line of the foregoing table represents is illustrated schematicallyl in FIGURE 6. Curves a an assumed depth of reflecting bed 16, which depths into Ztlf of FIGURIE 2 are reproduced, and curves 25a to crease downwardly in the table. For a .given assumed 40 25f of FIGURE 5 are reproduced as respective curves velocity, the differences in travel times can easily 'be com- 2Sa to 25)" of reversed polarity. The 12 curves of FIG- puted by simple trigonometry. It is evident that the URE 6 are then summed and reduced in amplitude by angularity of path corrections become less at greater a factor of one-twelfth to provide curve 28. It should depths because the diierence between the total travel be evident that this process is equivalent to that previoustimes becomes less. ly described so that curve 28 corresponds to curve 27.

With regard to the records illustrated in FIGURE 2, As previously mentioned, the manipulative steps of this it will be assumed that the corrections shown in the thrinvention can advantageously be performed by reproducteenth line of the table are proper to align the primary ing and combining the records with a magnetic tape or reflections. As previously discussed, the selection of drum recorder. This permits a number of trial and these particular corrections is generally a trial and error runs to be madein a short time to determine the error procedure t0 obtain the best corrections. The correct displacements. However, the invention is not curves of FIGURE 2 are then displaced by the times illimited to any specific apparatus. It should be evident lustrated in FIGURE 3 so that curves 20a to 201 are rethat theV curves can be moved 'and combined manually produced as respective curves 21a to 21j. It can thus be by drafting procedures, if desired. Y seen that the reilections R now occur at the same time. The process of this invention substantially eliminates The Curves 210 t0 2U 0f FIGURE 3 are 111611 summed, multiple reflections from the final seismic record. The and reduced iu amplitud@ by a factor 0f ODC-SiXiih, i0 PIO- desired reflections are amplified so that they can be recvide the composite record of curve 22. It can thus be ognized more readily. As previously mentioned, it is seen that the primary reilections are emphasized Whereas necessary in the practice of this invention to make an the multiple reflections are reduced in magnitude. arbitrary assumption as to the angularity of path correc- The next step in the process of this invention involves tions to be applied. However, a series of assumptions Combining the Original reCOrdS 0f FIGURE 2 in Such a are made until the best possible curve is obtained. manner that the multiple reflections are superimposed. While the invention has been described in conjunction In order to accomplish this result, it is assumed that the with present preferred embodiments, it shouldibe evident angularity of path corrections shown in the seventh line that it is not limited thereto. of the table are proper. The curves 20a to 20f of FIG- What is claimed is; URE 2 are displaced by the times illustrated in FIGURE l. The method of seismic surveying which comprises 4 to provide respective curves 23a to 231. These curves imparting to the earth acoustic vibrations at a plurality of are thenl summed and reduced in amplitude by a factor spaced lirst points and making ilrst records of reections of one-sixth to provide curve 24. The multiple reflecof said vibrations from a common subsurface area at a tions are ampl1ed in .this curve 24 of FIGURE 4, Whereplurality of second points respectivelyspaced from said as the primary reilections are reduced in amplitude. rst points, said first records containing primary reilec- The next step involves recording curve 24 six separate tions from said common subsurface reflecting area and urnes after being displaced amounts equal to the ditfermultiple reflections from reflecting areas above said com-y ences between the two angularity o f path corrections mon subsurface area, combining said first records with first portions of the first records which appear to be records of primary reflections from said common subsurface area substantially coincident so as to make a cornposite second record wherein the rst portions of the resulting composite record remain substantially intact and the remaining portions are substantially neutralized, said combining being accomplished by producing a plurality of third records, wherein said primary reflections occur at the same time relative to one another, which represent said first records displaced with respect to one another by preselected first amounts, producing a plurality of fourth records, wherein said secondary reflections occur at the same time relative to one another, which represent,

said first records displaced with respect to one another by preselected second amounts, summing said fourth records to form a fifth record, producing a plurality of sixth records by displacing said fifth record a plurality of times from a reference point by amounts equal to the respective differences between said second and first amounts, summing said third records, summing said sixth records, and subtracting the sum of said sixth records from the sum of said third records to obtain said composite second record.

2. The method of seismic surveying which comprises imparting to the earth acoustic vibrations at a plurality of spaced first points and making first records of reflections of said vibrations from a common subsurface area at a plurality of second points respectively spaced from said first points, said first records containing primary reilections from said common subsurface reflecting area and multiple reflections from reflecting areas above said common subsurface area, and combining said first records with first portions of the first records which appear to be records of primary reflections from said common subsurface area substantially coincident so as to make a composite second record wherein the first portions of the resulting composite record remain substantially intact and the remaining portions are substantially neutralized, said combining being accomplished by producing a plurality of third records, wherein said primary reflections occur at the same time relative to one another, which represent said first records displaced with respect to one another by preselected lirst amounts, producing a plurality of fourth records, wherein said secondary reflections occur at the same time relative to one another, which represent said first records displaced with respect to one another by preselected second amounts, sumrning said fourth records to form a fifth record, producing a plurality of sixth records by displacing said fifth record a plurality of times from a reference point by amounts equal to the respective differences between said second and first amounts, summing said third records, summing said sixth records, subtracting the sum of said sixth records from the sum of said third records to obtain said composite second record, and re peating the displacement, summing and establishing steps set forth above with different preselected amounts until the primary reflections in said composite second record are a maximum and the multiple reflections in the composite second record are a minimum.

3. The method of seismic surveying which comprises imparting to the earth acoustic vibrations at a plurality of spaced rst points and making first recordsk of reflections of said vibrations from a common subsurface area at a plurality of second points respectively spaced from said first points, said first records containing primary reflections from said common subsurface reflecting area and multiple reilections from reflecting areas above said common subsurface area and combining said first records with first portions of the first records which appear to be records of primary reflections from said common subsurface area substantially coincident so as to make a composite second record wherein the first portions of the resulting composite record remain substantially intact and the remaining portions are substantially neutralized, said combining being accomplished by producing a plurality of third records, wherein said primary reflections occur at the same time relative to one another, which represent said first records displaced with respect to one another by preselected first amounts, producing a plurality of fourth records, wherein said secondary reflections occur at the same time relative to one another, which represent said first records displaced with respect to one another by preselected second amounts, summing said fourth records to form a fifth record, producing a plurality of sixth records by displacing said fifth record a plurality of times from a reference point by amounts equal to the respective differences between said second and first amounts, summing said third records, summing said sixth records, and subtracting the sum of said sixth records from the sum of said third records to obtain said composite second record.

References Cited in the file of this patent UNITED STATES PATENTS 2,243,730 Ellis May 27, 1941 2,732,906 Mayne Jan. 31, 1956 2,882,988 Dobrin Apr. 21, 1959 2,888,089 Piety May 26, 1959 2,907,400 Swafford Oct. 6, 1959 

